In recent years, study and development of light-emitting diodes (hereinafter, referred to as “LEDs”) have rapidly advanced. Accordingly, various types of LEDs are developed, commercialized, and beginning to be used in wide range of fields. The LEDs are now being used in the field of lighting as well. Currently, in the field of lighting, the LEDs are used, for example, as a lighting device for a backlight for a liquid crystal panel, display plates and electrical signboards of various types, and the like.
Generally, the lighting device used for the backlight for a liquid crystal panel is of a direct type in which a diffusion plate is formed of a plate having a certain thickness and area, a light source such as a fluorescent light is disposed below the diffusion plate, and a surface of the diffusion plate emits light by being directly irradiated with light from the light source or an edge-light type in which a light guiding plate is formed of a plate having a certain thickness and area, and a light source such as a fluorescent light and an LED is disposed on at least one side of the light guiding plate to cause a surface of the light guiding plate to emit light.
Of the direct type and the edge-light type, the direct type has a structure in which a certain gap, i.e., a certain distance is provided between the light source and the diffusion plate. If the distance is short, the outer shape of the light source may be projected on the diffusion plate to give unpleasant appearance and degrade the lighting quality. If a highly directional light source is used, uniform illumination light may not be obtained because the brightness of a portion of the diffusion plate right above the light source becomes extremely high to generate difference in brightness between the portion and other lighting areas. As a method of uniformizing the brightness, a large distance may be provided between the diffusion plate and the light source. However, the use of this method causes problems in that desired illumination light cannot be obtained because the entire area proportionally becomes darker as the distance becomes larger, the device cannot be made thin, and the like.
Because the direct type lighting device has such problems, the direct type lighting devices may be difficult to be employed depending on the use.
Because the direct type lighting device has the problems described above, the edge-light type lighting device is used instead of the direct type lighting devices and numerous lighting devices of this type have been proposed (see, for example, Patent Documents 1 to 3).
For example, an edge-light type lighting device is disclosed in Patent Document 1. The lighting device is configured by including a light-emitting diode, a light guiding plate having a size of about a postcard in which a light guiding part is formed to have a flat surface, and a reflecting mirror that reflects light from the light-emitting diode, installing the light-emitting diode on the flat surface of the light guiding plate, and covering the light-emitting diode with the reflecting mirror. Thus, the light emitted from the light-emitting diode is reflected by the reflecting mirror to be guided to the light guiding plate. With the lighting device, the light emitted from the light-emitting diode is efficiently taken into the light guiding plate.
A lighting device including a light source device formed of an LED and a light source rod, and a light guiding plate that guides the light emitted from the light source device is disclosed in Patent Document 2. The light source rod is formed of a prism array of a prescribed shape. The light source rod makes an irradiation target object irradiated with the light emitted from the LED via the light guiding plate to uniformize the brightness.
Patent Document 3 discloses a register guide lamp in which a plurality of light-emitting diodes are disposed on a light entering surface of a light guiding body at an equal interval, light from the light-emitting diodes is irregularly reflected by a reflector, and the irregularly reflected light causes plane emission of light by the light-emitting surface of the light guiding body, whereby a display body disposed facing the light-emitting surface of the light guiding body is illuminated.